Three strands ironed closely together

It is not uncommon when braiding hair or bread to intertwine three different strands. At the molecular level, however, synthetic knots have thus far been restricted to architectures accessible from two-strand braids. Danon et al. used iron ion coordination to guide three organic ligand strands to form a knot geometry with eight separate crossings.

Abstract

Knots may ultimately prove just as versatile and useful at the nanoscale as at the macroscale. However, the lack of synthetic routes to all but the simplest molecular knots currently prevents systematic investigation of the influence of knotting at the molecular level. We found that it is possible to assemble four building blocks into three braided ligand strands. Octahedral iron(II) ions control the relative positions of the three strands at each crossing point in a circular triple helicate, while structural constraints on the ligands determine the braiding connections. This approach enables two-step assembly of a molecular 819 knot featuring eight nonalternating crossings in a 192-atom closed loop ~20 nanometers in length. The resolved metal-free 819 knot enantiomers have pronounced features in their circular dichroism spectra resulting solely from topological chirality.